Mitt Romney’s opponents have spent weeks slamming the private-equity industry where he made his fortune, saying it profits from dismantling companies and laying off workers.

The industry’s trade group has pushed back. Private-equity firms are good for the country, it argues, because they make the economy run more efficiently and turn around ailing companies, creating jobs.

A class-action antitrust lawsuit against private equity’s most powerful players threatens to further damage the industry’s reputation and, perhaps by extension, Romney’s. It alleges that the firms colluded to drive down the prices paid in some of the most ambitious buyouts in corporate history.

Perhaps more significantly, it is forcing an industry known for its secrecy to hand over reams of internal documents and offer up its top executives for depositions. So far, some of the most sensitive information has been kept under seal.

The defendants include the biggest names in private equity: KKR, Blackstone, the Carlyle Group, TPG and Bain Capital, which Romney co-founded in 1984, and five others. The lawsuit covers deals inked during the industry’s most recent boom, around 2006 and 2007, several years after Romney left Bain. Many of these firms and the Private Equity Growth Capital Council ­declined to comment.

The antitrust case has been expanding in scope just as Romney is running for president, which in turn is putting the industry in the crosshairs. The release of Romney’s tax returns this week, for instance, renewed questions about the lower tax rates enjoyed by private-equity partners.

The lawsuit, filed in 2007, charged that the firms illegally colluded on nine deals. The judge has since allowed evidence to be gathered on 18 others. The suit targets some of the biggest buyouts ever, including the $44 billion buyout of power company TXU, the $21 billion purchase of hospital operator HCA and the $17 billion acquisition of Harrah’s Entertainment.

The defendants say the allegations are groundless. “At its heart, this case is an attack on the process by which private-equity firms acquire control of public companies,” wrote the ­defendants’ attorneys in a motion to dismiss the case. They say that private-equity firms regularly paid a large premium on many of the deals. The scale of the deals was so big that many firms had to be involved. Moreover, all the bids were accepted by the boards of the companies being acquired.

The period in question was a heady time for the private-equity industry. The deals grabbed headlines because they often involved companies with household names. And each deal seemed to be bigger than the last.

Many of these acquisitions involved several private equity firms banding together to form what was called a “club deal.”

The plaintiffs, which include a Detroit pension fund and other shareholders in the acquired companies, allege that the firms agreed not to compete or bid up the prices on each another’s deals. In some cases, the suit alleges, there were even “inferior sham bids.”

“In this conspiracy, the winners and losers are clear,” the plaintiffs wrote in their complaint. “The winners are the private-equity firms, management and the investment banks. . . . The losers are the shareholders, whose equity defendants acquired deceptively and on the cheap.”

In the HCA case, the plaintiffs allege that a consortium that included KKR and Bain Capital faced no competition, even though the $51 per share buyout price was “a small premium” of 17.8 percent over HCA’s share price. They say that KKR and Bain were able to win the shares because the other big private-equity firms had agreed not to compete with them.

The complaint says that the lack of competition cost shareholders more than $1 billion.

The judge turned away efforts to dismiss the case but has not ruled on its merits. The discovery portion of the case is slated to end this spring, just as the presidential race heats up.

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